Enthalpy of Hydration

Discussion Questions

What is the energy of hydration?

How is hydration energy related to lattice energy?

What is enthalpy of solvation?

Hydration

The formation of a
solution
involves the interaction of solute with solvent molecules.
Many different liquids can be used as solvents for liquid solutions, and
water is the most commonly used solvent. When water is used as the solvent,
the dissolving process is called hydration.

The interaction between water molecules and sodium ion is illustrated
as one of the diagram below. This is a typical ion-dipole interaction.
At the molecular level, the ions interact with water molecules from all
directions in a 3-dimensional space. This diagram depicts the concept
of interaction only.

The above diagram also display hydrogen-bonding, dipole-dipole,
ion-induced dipole, and dipole-induced dipole interactions.
In the absence of these interactions, solvation takes place due to
dispersion. Definitions of these terms are obvious from the
diagrams. The meaning of the words used in the term also hints the
nature of the interactions.

What is the energy of hydration?

Enthalpy of hydration, Hhyd, of an ion is the
amount of energy released when a mole of the ion dissolves in a large
amount of water forming an infinite dilute solution in the process,
Mz+(g) + mH2O ®
Mz+(aq)
where Mz+(aq) represents ions surrounded by water molecules
and dispersed in the solution. The approximate hydration energies of some
typical ions are listed here. The table illustrates the point that as the
atomic numbers increases, so do the ionic size, leading to a decrease in
absolute values of enthalpy of hydration.

The hydration energy of an ionic compound consists of two inseparable parts.
The first part is the energy released when the solvent forms a coordination
compound with the ions. This energy released is called the energy of
ligation, Hlig. The processes related to these
energies are shown below:

Mz+ + nL = MLnz+, Hlig
MLnz+ + solvent = MLnz+ (solution), Hdisp
The second part is to disperse the ions or hydrated ions into the solvent
medium, which has a dielectric constant different from vacume. This amount of energy
is called energy of dispersion, Hdisp.
Therefore,
Hhyd = Hdisp + Hlig.
This idea is brought up just to point out that the formation of aqua
complex ions is part of the hydration process, even though the two energies
are not separable. When stronger coordination is made between the ions and
other ligands, they replace the coordinated water molecules if they are
present. In the presence of NH3 molecules, they replace the
water of Cu(H2O)62+:
Cu(H2O)62+ + 6NH3® Cu(NH3)62+
+ 6H2O

How is hydration energy related to enthalpy of crystallization?

In the discussion of
lattice energy, we consider the ions separated into
a gas form whereas in the disolution process, the ions are also separated,
but this time into ions dispersed in a medium with solvent molecules
between ions. The medium or solvent has a dielectric constant.
The molar enthalpy of solvation, Hsolv, is the
energy released when one mole solid is dissolved in a solvent.
This quantity, the enthalpy of crystallization, and energy of hydration
forms a cycle. Taking the salt NaCl as an example, the following
relationship is obvious,
Hhyd = Hcrystallization + Hsolv
from the following diagram.

The term enthalpy of crystallization is used in this diagram instead of
lattice energy so that all the arrows point downward. Note that enthalpy
of crystallization Hcryst, and energy of crystallization,
Ecryst referr to the same quantity, and they are used
interchangably.

The energies of solvation for some salts can be positive values, in these cases
the temperatures of the solution decrease as the substances dissolve.
The solvation is an endothermic reaction. The energy levels of solids and
solutions reverse in order of hight. The cycle is shown below.

In these cases, the enthalpies of hydration are less negative than the
enthalpies of crystallization.

What is enthalpy of solvation?

The molar enthalpy of solvation, Hsolv, is the
energy released when one mole solid is dissolved in a solvent.

Note that Hsolv is also called molar heat of solution
or molar energy of solution in some literature. Sometimes the
enthalpy of hydration is also (mis)understood as Hsolv.
When apply these values, make sure you understand the process involved.
The following enthalpies of solvation are given in Chemistry
by Radel and Navidi, West Publishing Co.

Enthalpy of Solvation (HsolvkJ/mol) of Some Common Electrolytes

Substanc

Hsolv

Substance

Hsolv

AlCl3(s)

-373.63

H2SO4(l)

-95.28

LiNO3(s)

-2.51

LiCl(s)

-37.03

NaNO3(s)

20.50

NaCl(s)

3.88

KNO3(s)

34.89

KCl(s)

-17.22

NaOH(s)

-44.51

NH4Cl(s)

14.77

These values indicates that when aluminum chloride and sulfuric acid
are dissolved in water, much heat is released. Due to the very small value
of enthalpies of solvation, the temperature changes are hardly noticed
when LiNO3 and NaCl are dissolving.

Example 1
The lattice energy of NaCl calculated from a physics view point using the
Madlung constant of the NaCl structure type is 788 kJ/mol. The estimated
enthalpy of hydration for sodium and chloride ions are 406 and 363 kJ/mol
respectively. Estimate the enthalpy of solvation for NaCl.

Solution

Using the cycle shown above and the formula, we have

Hhyd = Hcrystallization + HsolvHsolv = -769 - (-788) kJ
= 19 kJ/mol.

Discussion
A positive value indicates an endothermic reaction.
However, the value is small, and depending on the source of data, the
estimated value may change. This value of 19 kJ/mol is too high compared
to the value given earlier for NaCl of 3.88 kJ/mol, due to a high value
of lattice energy used.

Example 2
The
enthalpy of crystallization for KCl is -715 kJ/mol.
The enthalpies of hydration for potassium and chloride are -322 and
-363 kJ/mol respectively. From these values, estimate the enthalpy of
solvation for KCl.

Discussion
The enthalpy of solvation given above is -17.22.
The two values here indicates that the solvation is an endothermic reaction
or change. Should temperature decrease or increase when KCl dissolves?

Confidence Building Questions

What types of interaction are present when CaCl2 dissolves in
ethanlol?

Skill -
Identify the type of interactions in the solvation process.

What is the major type of interaction when toluene dissolves in benzene?

Discussion -
Toluene and benzene form ideal solution in that total vapor pressure of the
solution is the sum of the partial pressures of benzene and toluene. These
two compounds are so much a like that their molecules disperse into each
other. The major driving force for solution is entropy.

Which ion has a larger absolute value of enthalpy of hydration, Na+
or Ca2+?

Skill -
The ion-dipole interaction is stronger between Ca2+ and water
molecules than between Na+ and water molecules.

Which ion is larger, Na+ or Cl-?

Discussion -
What are their electronic configuration? The sodium ion has the same
electronic configuration as Ne, but the chloride ion has the same electronic
configuration as Ar, which is larger than Ne. Which one release more energy
when hydrated?

When KNO3 is dissolving in water, will the temperature decrease
or increase?

Discussion -
Among the substances listed in the table above, KNO3 absorbed
the most energy per mole when dissolved.